Process and device for assessing electroencephalograms

ABSTRACT

In order to facilitate assessment of the activity of the brain by the medical practitioner, the description relates to a process for assessing electrically recorded brain activity in which a signal taken from a skull over a predetermined period at a predetermined scanning frequency is converted into a sequence of data words from which the amplitudes and the frequencies relating to each amplitude are extracted in a computer system in a predetermined pattern in such a way that intermediate data words are formed, each of which represents an amplitude and is stored under a store address determined by the frequency allocated to the intermediate data word, and those maximum intermediate data words of all of them are determined which are a relative maximum and a mean is drawn at each frequency from the maximum intermediate data words and all means with the relevant frequency are taken to a display device for representation in a frequency/amplitude curve.

The invention relates to a method for evaluating electrical braincurrent signals picked off from the top of the skull over a given timeinterval, and to a device which is suitable for this purpose.

German Laid-Open Specification 15 41 173 discloses a method forinformation compression of electroencephalograms, time intervals beingdetermined for which the upper and lower curve inversion point of abrain current signal is the governing factor. German Laid OpenSpecification 22 47 572 discloses a method and an apparatus forautomatic analysis of brain current signals, in which method a signal isproduced from an analog EEG signal over a given time interval, that sic!relates to the frequency of the EEG signal. These methods for braincurrent representation have the disadvantage that they do not providethe doctor with any cohesive, simple, easily viewed information aboutthe detected brain activity.

EP-A-0 150 125 discloses a method for evaluating electrically detectedbrain activity, in which a signal which is picked off from the top ofthe skull over a predetermined time interval is converted at apredetermined sampling frequency into a sequence of data words fromwhich the amplitudes as well as the frequency associated with eachamplitude are extracted in accordance with a predetermined scheme, forexample a Fourier transformation, in an arithmetic unit, in such amanner that intermediate data words are formed, each of which representsan amplitude and is stored at a memory address. Furthermore, mean valuesare formed from the intermediate data words in the known apparatus, andall the mean values are passed, with the associated frequency, to anindicator device for display on a frequency/amplitude graph.

The invention is thus based on the object of making it easier for thedoctor to assess brain activity.

To this end, the invention provides that the signal is converted at agiven sampling frequency into a sequence of data words from which theamplitudes and the frequency associated with each amplitude areextracted in accordance with a predetermined scheme (Fouriertransformation or the like) in an arithmetic unit, in such a manner thatintermediate data words are formed, each of which represents anamplitude and is saved at a memory address which is governed by thefrequency associated with the intermediate data word, furthermore, ofall the intermediate data words, those maximum intermediate data wordsbeing determined which are a relative maximum, and in such a manner thata mean value is formed from the maximum intermediate data words for eachfrequency and all the mean values are passed with the associatedfrequency to an indicating device for display using afrequency/amplitude graph.

There is thus a so-called sum spectrum for each recorded track on theelectroencephalogram, from which the doctor can use the position of themaxima for the individual frequencies to draw conclusions about brainactivity directly. The basic activity and any pathological activity ofthe brain activity can thus be taken from a single curve profile, andcan easily be assessed medically.

In a preferred refinement of the invention, in order to form the meanvalue, the sum of the maximum intermediate data words present perfrequency is multiplied by the number of said words and is divided by agiven reference variable.

Furthermore, it is recommended that, with the time interval beingsubdivided into a number of time units, the extraction be carried outfor each time unit (sampling second), the intermediate data words formedfor each time unit being saved at memory addresses which are definedunambiguously for the time unit by the frequency associated with theintermediate data word, and the number of time units in the timeinterval being chosen as the reference variable. The mean value datawords fed to the indicating device (30) are preferably passed via alow-pass filter.

For deeper evaluation by the doctor and as a continuation of theinvention, it is possible to determine from the mean value data wordsthose which are a relative minimum, the frequency associated with eachminimum mean value data word being fed to the indicating device forseparate display. A frequency drift graph can be obtained from this, ifthe frequencies of the maximum intermediate data words in each time unitare determined and are passed to the indicating device.

A single graph is thus produced for one record track of the EEG, inwhich the doctor can immediately see the times during the scan, and thefrequencies, at which activity maxima have occurred. This considerablysimplifies medical evaluation, in comparison with conventionalrepresentation.

Finally, the invention makes it easy to check, for example, the Bergereffect if, for each time unit, all the intermediate data words within afrequency interval are passed to the indicator unit for an amplitudedrift display, the frequency interval being defined by the frequencydifference between second two minimum mean value data words.

A device as specified in claim 10 is particularly suitable for carryingout the described method.

In addition, preferred embodiments of the invention are contained in thedependent claims.

The invention will be described in detail in the following text withreference to the exemplary embodiment illustrated in the attachedfigure, in which:

FIG. 1 shows a block diagram of a device which is suitable for carryingout the method according to the invention;

FIG. 2 shows an electroencephalogram of the track FP₂ -F₈ recorded overa time interval of 10 s;

FIG. 3 shows a second spectrum of the seconds zero and one from therecord in FIG. 2;

FIG. 4 shows a second spectrum, similar to FIG. 3, from the seconds 2and 3 in the record in FIG. 2;

FIG. 5 shows a non-filtered sum spectrum from the record in FIG. 2, butover a time interval of 210 s;

FIG. 6 shows the filtered sum spectrum from FIG. 5;

FIG. 7 shows a frequency drift presentation from the record in FIG. 2;but over a time interval of 210 s;

FIG. 8 shows a frequency drift presentation similar to FIG. 7 but from adifferent track, and

FIG. 9 shows an amplitude drift presentation.

The exemplary embodiment of the invention described in the followingtext will be used to illustrate only those medical phenomena which aremanifested in the frequency range up to 16 Hz. In anelectroencephalogram, the brain currents from 20 signal channels arenormally presented. For simplicity, the invention will be explainedusing the record of predominantly only one channel, for example in thetrack FP₂ -F₈. The invention can, of course, be applied to all trackssince the point of origin of the signals, that is to say the specifictrack, is irrelevant for the signal processing described in thefollowing text. The doctor compares the presentation of the signal withthe recorded track, and draws medical conclusions from this.

Electrodes are attached in a normal manner to the top of the skull of apatient, at the pick-up points FP₂ and F₈, and the electrical leads 3, 4connected to the electrodes are connected to the associated inputs of anamplifier 5.

In a conventional display, the amplifier would cause a curve 8 to bedisplayed on a downstream indicator unit, which is not illustrated, asis shown in FIG. 2 for the first 10 s of the record. The complete timeinterval of the record would be about 210 seconds, that is to say about3.5 minutes. The time units associated with the first 10 seconds areindicated in FIG. 2 by S 0 . . . S 9.

The amplified analog signal 8 is fed from the amplifier 5 via an outputline 6 to an analog/digital converter 10 which converts the recordedanalog signal into 8-bit data words. To this end, the analog/digitalconverter 10 contains a sample generator which samples the signal,arriving in analog form, at a sampling frequency of 128 Hz, the dataword being formed for each sample from the amplitude value of the analogsignal. Thus, a data word appears for each sampling time and at afrequency of 128 Hz on the 8-wire output line 12, is fed in bit-parallelform to the input connection 14 of an evaluation device 15 and contains,in coded form, the amplitude value of the sampled, amplified signal 8.

The evaluation device 15 has a central processor 18, a first memory 24which may be a hard disk memory, a program memory 22, a second memory 20as a read/write memory and a video interface 26 whose inputs and outputsare connected to an address and control bus 16. The video interface 26has a further output line 28, which leads to an indicating device 30having a screen.

Controlled by the central processor 18, the bit-parallel data wordspassed in serial form from the output line 12 to the address and controlbus 16 are saved at successive addresses in a first memory 24. Eachaddress in the first memory thus represents the sampling time at whichthe data word was generated by the analog/digital converter 10.

A transformation program, which extracts the associated frequencies andamplitudes from a stream of data words, is stored in the program memory22. Such a program may represent, for example, a Fourier transformation.In the present case, the program represents the maximum entropy methodas is explained, for example, by W. H. Press et al. in "NumericalRecipes of the Art of Scientific Computing", Cambridge University Press,1988, pages 572-576.

Once all the 26,880 data words generated over the recording timeinterval of 210 seconds have been saved in the first memory 24, they aretransformed section-by-section under the control of the centralprocessor 18 and with the aid of the scheme stored in the program memory22, and the result of the transformation is saved in the second memory20 at the addresses assigned to the second S 0.

Saving section-by-section in this case means that all 128 data wordsproduced during the second S 0 (FIG. 2) are transformed and saved firstof all, that the data words produced during the second S 1, for example,are transformed, and are saved at the address assigned to the second S1, in the second step, as so on. A set of intermediate data words notedin the second memory is thus obtained from the set of data words.

In the second memory 20, the addresses assigned to the sampling second S0 are unambiguously assigned to the frequencies f=0 Hz . . . f=16 Hz.For example, an intermediate data word which is associated with thefrequency 1 Hz and is obtained from the transformation of the data wordsfrom S 0 is saved in the second memory, at the address associated withthe frequency 1 Hz. The intermediate data words which result from thetransformation of the data words from the sampling second S 1 are savedin a corresponding manner in the second memory at addresses which onceagain correspond unambiguously to the frequencies f=0 . . . 16 Hz, butnow for S 1, to be precise for each intermediate data word at theaddress associated with its frequency. Each intermediate data wordcontains a coded value proportional to an amplitude. An address list, towhich the central processor 18 has access, is for this purpose saved ata suitable point in one of the units 18, 20, 22, this address listcontaining the addresses associated with the frequencies for each of the210 sampling seconds.

If, for example, it is now desired to display the content of the secondmemory in an amplitude/frequency graph on the indicating device 30, theseconds spectra illustrated in FIG. 3 would be obtained for the secondsS 0 and S 1, and the seconds spectra illustrated in FIG. 4 for theseconds S 2 and S 3.

As indicated in FIGS. 3 and 4, the seconds spectra for the seconds S 0and S 2 are illustrated by solid lines, and those for the seconds S 1and S 3 by dashed lines. It can be seen that, in S 0, the curve 32 hasan amplitude maximum at about 5.2 Hz, and a further, higher amplitudemaximum at about 7.5 Hz. In S 1, the curve 34 shows an amplitude maximumat about 4.5 Hz and a very much higher amplitude maximum at 8 Hz. For S2, the curve 36 shows an amplitude maximum at about 5 Hz, and a furtherat 8 Hz, and the curve 38 for S 3, finally, has an amplitude maximum atabout 4.7 Hz and a small maximum at about 8 Hz. There is also a thirdmaximum in the seconds spectra for the seconds 2 and 3 in the region of11.4 Hz.

In order to improve the evaluation of the analog signal S, particularlyin the boundary regions between the individual seconds in the samplingtime interval, the sections of data words on which the transformation iscarried out can be designed such that these sections cover the saidboundary regions.

In order to obtain a first medically meaningful representation of therecord according to the analog signal 8, a sum spectrum is produced fromthe contents of the second memory in the following manner: first of all,the magnitudes of the intermediate data words for each second section inthe second memory, that is to say for each address section relating to S1 . . . S 210, are compared to determine those intermediate data wordswhose value is greater than that of the two adjacent intermediate datawords. In other words, the central processor 18 determines the amplitudemaxima in each second section. Each maximum intermediate data worddetermined in this way is marked by the central processor 18, forexample by setting an additional flag bit in the intermediate data word.The following averaging process is then carried out in an arithmeticunit (which is not illustrated separately) in the central processor 18for each frequency to which an address in the second memory area isassigned: the sum of all the amplitudes of the marked intermediate datawords associated with one frequency is multiplied by the number ofmarked intermediate data words associated with this frequency and isdivided by the number of sampling seconds (210). In a third memory areaof the read/write memory 20, the mean value data words obtained in thisway are saved at memory addresses which are unambiguously assigned tothe frequency. The mean value date words and signals corresponding totheir addresses from the third memory area of the read/write memory 20are passed under the control of the central processor 18 via the videointerface 26 to the indicating device 30. A curve 40, which is shown inFIG. 5, appears on the associated screen, as a amplitude/frequencygraph. The amplitude value of the associated mean value data wordappears on this curve, for each frequency.

As can be seen from FIG. 5, the curve 40 is distinguished by a largenumber of peaks, some of which are very sharp. To make it easier for thedoctor to evaluate this, the middle data words from the third memoryarea are passed via a low-pass filter, and are only then fed via theaddress and control bus 16 to the video interface 26, as a result ofwhich the curve 40 is smoothed to the sum curve 42 illustrated in FIG.6. Apart from a DC element on the branch 41 of the curve 42 in thefrequency band below 1.5 Hz, the observer sees a first relative maximum44 at about 4 Hz and a, smaller, second maximum 46 at about 8 Hz. Abarely perceptible third maximum 50 may be noted at about 15 Hz.

Medically, this result means that the patient investigated hasconsiderable brain activity at 4 Hz, and less at 8 Hz. Above all, thefirst maximum 44 is significantly lower than the frequency band of 8-13Hz of alpha waves, which indicates a pathological situation in the brainsection of the patient, which corresponds to the position recorded.

In order to increase even further the usefulness of the presentation ofthe analog signal 8 for the clinician, a frequency drift display of theanalog signal is produced with the assistance of the data words storedin the first memory 24, and this allows the extracted frequencies to becompared with the medical division into the already mentioned alphawaves (frequency band from 8 to 13 Hz), the beta waves (frequency above13 Hz), the delta band (frequencies below 3 Hz) and the theta band(frequencies of 4-7 Hz).

Specifically, as can be seen from FIG. 6, increased brain activity,represented by the maxima 44, 46 and 50 takes place within the frequencybands whose boundaries are indicated by the minima located between themaxima.

For example, there is a relative minimum 43 at about 2.5 Hz, a furtherminimum 45 at about 6.2 Hz, and a third minimum 47 at about 11 Hz. Thecentral processor 18 thus, for example, uses the already mentionedcomparitor to find from the third memory area in the read/write memory20 the frequencies at which relative minima may occur in the mean valuedata words.

The associated frequencies F_(min) and the signals corresponding to themare passed via the video interface 26 to the indicating device 30 andare displayed as horizontal lines 53, 55, 57 on the screen in a graphwhose abscissa is the overall recording time interval in seconds andwhose ordinate is the frequency in Hz.

Furthermore, the central processor 18 accesses the second memory 20 and,within each second section, checks all the intermediate data words forthe presence of the marking (which, as mentioned, indicates a relativemaximum). The addresses associated with the marked intermediate datawords (which represent a frequency) are plotted on the graph in FIG. 7as (frequency) points associated with the respective second section, byfeeding the respective signals to the indicating device 30. Theindicator unit 30 thus receives one or more frequency signals for eachsecond section S 0, S 1 . . . S 210 in the second memory area, each ofwhich is allocated to an intermediate data word representing anamplitude maximum. The indicating device 30 enters these frequencysignals on the graph illustrated in FIG. 7. A number of interruptedcurve families can be seen within the individual frequency bands,indicated by the dashed lines 53, 55, 57 over the recording timeinterval of 210 seconds.

To improve the clarity particularly in the region of the basic rhythm inthe graph in FIG. 7, the frequency signals that occur in this frequencyband are passed separately via a low-pass filter, and the output signalfrom the low-pass filter is plotted by the indicating device in thefrequency range between the curves 55 and 57.

This results in the integration curve 59 illustrated. The basic rhythmis defined by that frequency interval between the curves 53, 55, 57 inwhich the number of frequency signals that occur is a maximum.

From the frequency drift graph for the track FP₂ -F₈ in FIG. 7, thedoctor can see that the basic rhythm associated with the alpha wave bandis, in general, slightly slowed down on the part of the hemisphere ofthe patient being detected, since the basic rhythm of the brain activityis in this case in the frequency range from about 6.5 to about 10.5 Hzand the integration curve is very frequently under 8 Hz while the basicrhythm in a healthy person may be expected in the alpha band from 8 to13 Hz. Furthermore, the doctor can see from this frequency drift graphthat the patient being examined presents continuous epileptogenic fociof medium severity in the right, temporal lobe. In another track T3-T5(FIG. 8), it can be seen that there is no activity in the region from 4to 7 Hz. The basic rhythm has also been slightly slowed down, and theactivity in the region below 4 Hz may be ascribed to eye-movementartifacts. By comparing the frequency drift graphs for different brainregions, the doctor can distinguish between a focus and a generalizedphenomenon.

Finally, the contents of the second memory can also be presented asamplitude drift, which allows the doctor to make further deductionsabout the brain activity. To do this, the central processor 18 onceagain accesses the second memory 20 and determines second-by-second,that is to say first for S 0 then for S 1 etc., those intermediate datawords which are within a given frequency range, that is to say a givenaddress subsection.

Each address subsection is defined by the frequency interval between tworelative minima in the sum spectrum in FIG. 6, that is to say by thefrequency boundaries 53, 55, 57 in FIG. 7. In the case of an EEG recordon the track T3-T5 (FIG. 8), the frequency intervals are at 3.8 . . .11.9 Hz and at 11.9 . . . 12.8 Hz. For each of the intermediate datawords already found in each frequency interval and having a marking(maximum) the central processor 18 forms a signal proportional to thevalue of this word and sends this signal to the indicating device 30.The indicating device 30 enters these signals on a graph according toFIG. 9, whose abscissa once again shows the total recording timeinterval of 210 seconds and whose ordinate shows the amplitudes in μvolts, for each frequency range with respect to a specific zero line.The family of curves 64 then appears over the entire recording timeinterval (S 1 . . . S 20) over the reference line 62 relating to thefrequency band 11.9 . . . 12.8 Hz. This family of curves 64 shows, at66, 68 and 70, so-called amplitude suppressions, which are characterizedby amplitude values of virtually zero μ volts. A comparison of thetiming of the amplitude suppressions 66, 68 and 70 with the record showsthat the patient opened his eyes during the associated recording times.The triple Berger effect at the recording times of 30, 70 and 140seconds may thus medically be assessed as being positive.

Finally, the amplitude drift graph in FIG. 9 shows high-amplitudeeye-movement artifacts 72 in the frequency band from 3.8-11.9 Hz at therecording times from 140-150 s.

It can be seen that the type of processing for EEG signals describedabove using the evaluation device 15 and the analog/digital converter 10allows displays of brain activity to be obtained in the indicatingdevice 30 which allow the doctor to make a medical assessment easily anddirectly.

Frequency-linked phenomena, such as the basic rhythm, pathologicallyslow activity or specific artifacts are described individually inseparate frequency bands, can be evaluated individually by the doctorand do not interfere with one another as in a conventional EEG.

I claim:
 1. Method for evaluating electrically detected brain activity,in which a signal picked off from the top of the skull over a given timeinterval is converted at a given sampling frequency into a sequence ofdata words from which the amplitudes and the frequency associated witheach amplitude are extracted in accordance with a predetermined scheme(Fourier transformation or the like) in an arithmetic unit, in such amanner that intermediate data words are formed, each of which representsan amplitude and is saved at a memory address which is governed by thefrequency associated with the intermediate data word, furthermore, ofall the intermediate data words, those maximum intermediate data wordsbeing determined which are a relative maximum, and in such a manner thata mean value is formed from the maximum intermediate data words for eachfrequency and all the mean values are passed with the associatedfrequency to an indicating device (30) for display using afrequency/amplitude graph.
 2. Method according to claim 1, characterizedin that, in order to form the mean value, the sum of the maximumintermediate data words present per frequency is multiplied by thenumber of said words and is divided by a given reference variable. 3.Method according to claim 1, characterized in that each mean value istemporarily saved, as a mean value data word, at an address governed bythe associated frequency.
 4. Method according to claim 1, characterizedin that, with the time interval being subdivided into a number of timeunits, the extraction is carried out for each time unit (samplingsecond), the intermediate data words formed for each time unit beingsaved at memory addresses which are defined unambiguously for the timeunit by the frequency associated with the intermediate data word, and inthat the reference variable is the number of time units in the timeinterval.
 5. Method according to claim 1, characterized in that the meanvalue data words fed to the indicating device (30) are passed via alow-pass filter.
 6. Method according to claim 1, characterized in that,of the mean value data words, those are determined which are a relativeminimum, and in that the frequency associated with each minimum meanvalue data word is fed to the indicating device (30) for separatedisplay.
 7. Method according to claim 1, characterized in that thefrequencies of the maximum intermediate data words in each time unit aredetermined and are passed to the indicating device (30) to provide afrequency drift display.
 8. Method according to claim 7, characterizedin that the frequencies which are in the region of the basic rhythm arepassed via a low-pass filter, and the output of the low-pass filter isfed to the indicator unit (30).
 9. Method according to claim 1,characterized in that, for each time unit, all the intermediate datawords within a frequency interval are passed to the indicator unit (30)to provide an amplitude drift display, the frequency interval beingdefined by the frequency difference between two minimum mean value datawords.
 10. Device for evaluating electrically detected brain activityfor carrying out the method according to claim 1, having ananalog/digital converter (10) in which a signal (8) picked off from thetop of the skull over a given time interval is converted at a givensampling frequency into a sequence of data words, having an evaluationdevice (15) whose address and control bus (16) feeds the data words fromthe analog/digital converter (10) to a first memory (24), apredetermined scheme (Fourier transformation or the like) being storedin a program memory (22) in the evaluation device (15), by means ofwhich scheme a central processor (18), which is provided in theevaluation device (15) and contains an arithmetic unit, extracts fromthe data words stored in the first memory (24) the amplitudes as well asthe frequency associated with each amplitude and saves the results in asecond memory (20) in such a manner that the addresses which areassigned to the frequencies have intermediate data words written tothem, each of which represents an amplitude, the arithmetic unitdetermining those intermediate data words which are a relative maximumand forming a mean value for each frequency from the maximumintermediate data words, and the central processor (18) feeding all themean values, with the associated frequencies, via the address andcontrol bus (16) to an indicating device (30).
 11. Device according toclaim 10, characterized in that the indicating device (30) has a screenand is coupled to the address and control bus (26) via a video interface(26).
 12. Device according to claim 10, characterized in that, in orderto form the mean value, the arithmetic unit multiplies the sum of themaximum intermediate data words present per frequency by the number ofsaid data words, and divides it by a given reference variable. 13.Device according to claim 10, characterized in that the centralprocessor (18) temporarily saves each mean value as a mean value dataword at an address, governed by the associated frequency, in a thirdmemory.
 14. Device according to claim 10, characterized in that theevaluation device (15) has a low-pass filter via which the centralprocessor (18) feeds the mean value data words to the indicating device(30).
 15. Device according to claim 10, characterized in that thearithmetic unit determines from the mean value data words those whichare a relative minimum, and in that the central processor (18) feeds tothe indicating device (30) the frequency associated with each minimummean value data word.
 16. Device according to claim 10, characterized inthat, with the time interval being split into a number of time units,the arithmetic unit carries out the extraction for each time unit(sampling second) and the central processor (18) saves the intermediatedata words formed for each time unit at memory addresses in the secondmemory (20), which is are unambiguously defined for the time unit by thefrequency associated with the intermediate data word, and in that thereference variable is the number of time units.
 17. Device according toclaim 10, characterized in that the arithmetic unit passes, for eachtime unit, all the intermediate data words within a frequency intervalto the indicating device (30), the frequency interval being governed bythe frequency difference between two minimum mean value data words. 18.Device according to claim 10, characterized in that the centralprocessor (18) has a list of memory addresses with associatedfrequencies.
 19. Device according to claim 10, characterized in that thearithmetic unit passes to the indicating device (30), for each time unitand via a low-pass filter, all those frequencies which are within afrequency interval associated with the basic rhythm.